A question on penetration speed.

[Anonymous]

I have seen many times people have been told to "slow down" their sweep speed. I want to ask this in a correct manner as I a very curious for the answer. Does it really take that long for a radiated pattern to penetrate the ground deeply and then receive a return signal...I mean can you swing the loop so fast that you actually can pass over a target without the detector given a beep? I guess I can lay a target down on the ground and pass the coil over it and get the answer but is this a truthful answer to the question? With a fast sweep speed the radiated pattern is still there, not distorted as air could not distort a electro magetic field...so in fact the field is still there and the circuits do not pick up on it? I may be asking a stupid question but if the field will penetrate the ground at a slow sweep speed why would a faster sweep speed not allow it to penetrate the ground?
Hope this is not to elementry a question...Frank

 

[Anonymous]

Hi Frank,
Electromagnetic waves propagate at the speed of light, and this exactly what happens with any metal detector. A coin or ring at 15in is going to see the transmitter field virtually instantaneously, and likewise the return signal is at the same speed, so there is no problem with sweep speed here.
For a small object at that range, the return signal is very minute and will probably be at the microvolt level. On the other hand, electromagnetic noise picked up by the coil from power lines, radio transmissions and general atmospheric noise can be many times higher than the object signal. Some sort of signal processing then has to be done to extract the object signal from the noise and the simplest way of doing this is by averaging over a small period of time. In the process of averaging the noise, being to some degree random or non-coherent with the transmitter frequency, will average to zero, whilst the object signal will be enhanced to a useable level. You may have to average over a thousand pulses to achieve this, and if you are transmitting 1000 pulses per second, then there is going to be an apparent lag in the detector responding as you pass the coil over an object. If you sweep too fast, then you could miss a deep target. Sweeping more slowly allows the electronics to catch up with the signals being processed.
One of the reasons why PI detectors (mine in particular) use high pulse rates, is so that the object signals are coming in faster and averaging can be done over a shorter time, thereby minimising the lag.
There is another reason for not sweeping too fast, and that involves the SAT, or Self Adjusting Tuning. It

 

[Anonymous]

Sir, I do appreciate your answer and I am so pleased you explained it to me in a manner that I understood, I know you are capable of slinging those words way over my head, I have saved this to the disk for future referance.
Before your response I had passed a US Nickel and gold ring by the coil in extremelly fast speed and knew that it 'would not' miss making a beep, it is the "other" varibles/outside interference that you have to battle as an engineer and designed, that I had not considered.
Again I appreciate your time and your time used explaining this to me. Everyday is a new learning experience, you can never know enough or all...Frank